Charged particle gun with nonspherical emissive surface



June 30, 1964 BREWER 3,139,552

CHARGED PARTICLE GUN WITH NON-SPHERICAL EMISSIVE SURFACE 3 Sheets-Sheet1 Filed March 7, 1960 5y i r W haw? Arm/#04 .N @NNN. g H\ N W June 30, 4s. R. BREWER 3,139,552

CHARGED PARTICLE GUN WITH NON-SPHERICAL EMISSIVE SURFACE WT W June 30,19 4 cs. R. BREWER 3,139,552

CHARGED PARTICLE GUN WITH NON-SPHERICAL EMISSIVE SURFACE.

Filed March 7, 1960 3 Sheets-Sheet 3 United States Patent 3,139,552CHARGED PARTICLE GUN WITH NON- PHERICAL EMISSIVE SURFACE George R.Brewer, Los Angeles, Calif, assignor to Hughes Aircraft ompany, CulverCity, Calif, a corporation of Delaware Filed Mar. 7, 1960, Ser. No.13,333

3 Claims. (Cl. 313-83) This invention relates to devices for formatinghigh density charged particle beams, and more specifically to animproved particle gun for producing a collimated beam having an improveduniformity of particle density across its cross section.

In recent years, a number of new types of microwave electron tubes, suchas traveling-wave tubes, have been developed. This invention is notlimited to electron devices per se but encompasses devices utilizingother charged particles as well; however, henceforth in thisspecification the term electron will be used and should be taken toinclude, where applicable, other charged particles.

Amplification in a traveling-wave tube involves an accumulativeinteraction between an electromagnetic wave and an electron beam movingin a predetermined relationship With respect to the wave. Thisspecification is concerned in part with some of the details of theproduction of this electron beam and will have occasion to refer tobeams in terms of perveance, which is defined as the ratio of total beamcurrent to the three-halves power of the beam voltage. For usefulperformance and interaction between the electron beam and a travelingwave, a traveling-wave tube requires a minimum perveance of the order ofone hundred times that in the conventional cathode ray tube beam suchas, for example, in an oscilloscope tube or television picturereproducing tube. In higher power traveling-wave tubes the perveancerequired is up to one thousand times this minimum value that is 1 tox10- amperes/volt 3/2. Because of such a high value of perveance, whichis a measure of the tendency for beam spreading due to space chargerepulsion of the electrons, beams for use in traveling-wave tubes mustbe provided with some type of focusing or constraining means tocounterbalance the space charge force of the electrons in order toobtain a reasonably smooth, constant diameter beam. This is necessary inpart because it is desired for maximum interaction that the electronstream travel along the interaction or slow-wave structure of thetraveling-wave tube as closely thereto as possible. Such a high powerbeam if not well focused would be intercepted by the structure and wouldimmediately damage or destroy the interaction structure. A common way ofaccomplishing this focusingis by immersing all or a part of the electronstream in a uniform magnetic field.

The gradual development of traveling-wave tubes to higher output powercapabilities has required the production of beams of higher current inwhich the beam current density frequently exceeds that available fromthe cathode. For example, cathodes may generally emit of the order offrom 10 to 20 amperes per square centimeter of surface, while therequired beam current density may be of the order of a few hundredamperes per square centimeter. A converging beam gun in which thecathode area is several times larger than the ultimate beam area hasbecome a commonly accepted means for forming the initial beam. Theelectron gun is usually disposed outside of the focusing magnetic fieldso that the initial beam formation results principally from electricfields in the gun. It is desired to design the shape of the gunelectrodes in such a way that the electric fields result in uniformemission and a well collimated beam resulting in a uniform currentdensity over the cross section of the beam. Such a gun is essentially adiode with a large concave 3,139,552 Patented June 30, 1964 emissivecathode emitting electrons toward an anode having a central aperture forpermitting the beam to pass therethrough toward the remainder of thetraveling-wave tube and into the environment of an axial focusingmagnetic field.

In the anode aperture transverse electric fields cause an outwarddeflection of the electrons. This effect is frequently treated byconsidering the anode aperture as an electrosatic lens with a certainfocal length. This is a thin lens concept and as an approximation isuseful in guns of low perveance, for example, less than 0.1 1()" amperesper volt 3/2. However, this approximation breaks down rapidly as the gunperveance is increased. This lens effect is important and theradius andslope of the beam emerging from the gun must be designed to becompatible with the beamperveance and the magnetic field system in orderto obtain optimum beam focusing in the region beyond the gun. It maytherefore be seen that the details of the electron gun design are ofcritical'im portance in order to obtain a satisfactory focused beam.

If the focal length of the anode aperture lens mentioned above could bemade constantwith respect to electrons entering the lens at varyingradial distances off axis, so that in the absence of any magnetic fieldall the electrons from the cathode would converge and focus to or towardone point, it would be comparatively straightforward to introduce thebeam of electrons into an environment of an axial magnetic field and toconstrain them to flow in a well collimated beam through thatenvironment. The type of flow is Brillouin flow in which the centrifugaland space charge forces on the spiraling electrons are balanced by theradially directed magnetic forces as the beam moves through thestructure. However, when a hole is cut in the anode to allow the beam topass through, a number of problems arise. If a grid is placed over theanode aperture so as tomaintain a unipotential surface or an effectivelycontinuous electrode, or if the diameter of this aperture is smallcompared with the anode-cathode distance, as in low perveance guns, theaperture may be ignored. However, in a high voltage gun, the powerdissipated in a grid from electron interception would normally cause thegrid to operate at a prohibitively high temperature; and as theperveance of a gridless gun is increased the anode aperture must be madelarger compared with the cathode-anode distance and the distortion inthe electrostatic field is increased. This field distortion is the majorcause of the variation in'focal length of the electric lens as seen byelectrons entering the lens at different radii and is analogous tospherical aberration in an optical lens. This concept and problem offield distortion will, in accordance with the terminology in the art, behereinafter referred to as aberration or spherical aberration in theelectron lens. This aberration in the electron lens gives rise to ahighly nonuniform current density distribution across the cross sectionof the electron stream. Electrons in the regions of higher density tendto have a greater component of radially inward velocity as they emergefrom the electron gun. They soon, after leaving the gun, approach theaxis where they subsequently begin to diverge because of their spacecharge repulsion forces. After expanding to a larger radius they areagain focused inwardly by the magnetic field and the processes repeatsalong the length of the traveling-wave tube causing the beam to bescalloped. The interaction structure of the tube must be placed at aradial distance from the axis represented by the maximum amplitude ofradial excursion of the scalloping electrons in order to precludemelting or other damage to the helix or other structure.

In the past, there have been a number of attempts to correct or tocompensate for this lens aberration. One approach has been to increasethe strength of the magnetic field above that required for theoreticalBrillouin flow.

This, however, is expensive as regards electromagnet power and weightrequirements in mobile or airborne equipment. Further, the solution isnot adequate because the stronger magnetic field does not cure thescalloping but, for the most part, merely changes its period and outerdiameter. Another approach is described in United States Patent2,811,667, issued to George R. Brewer, on October 29, 1957, entitledElectron Gun. The approach as described there is to utilize a secondanode downstream from the primary anode maintained at a higher directcurrent potential and designed to project an electrostatic field effectinto the cathode-anode region which compensates for the field distortionof the character above referred to. Another approach is to attempt tocorrect for the field distortion by utilizing additional electrodes inthe cathode-anode region itself or to shape the anode in a manner toattempt to eliminate the field distortion. These and other approachesare inadequate or suifer limitations in high perveance guns such as tocause them to be less than a complete solution in high perveance systemswhere a high degree of collimation and focusing is essential.

It is therefore an object of the present invention to provide anelectron gun which does not suffer the disadvan- ,tages of the priorart.

It is in particular an object to provide an electron gun to produce astream of charged particles which is well collimated and may be focusedover an extended path,

such as the length of a traveling-wave tube or the like.

The stream maybe cylindrical, hollow cylindrical, planar, or the like.

It is a further object to provide an electron gun which ,does not suifera serious degree of lens aberration.

It is a further object to provide a converging beam electron gun forproducing a high perveance or very high perveance electron stream whichmay be well focused and confined throughout an extended longitudinalbeam path.

Briefly, in accordance with the present invention, these ,and otherobjects are achieved by a new charged particle gun having a source witha curved emissive surface which is shaped in a manner to provide aconverging stream of particles which may be of extremely high densityand perveance. In one example, in which a very high perveancecylindrical beam of electrons is produced, the cathode surface is aconcave figure of revolution about the axis of a traveling-wave tube.The radius of curvature of the curved surface varies as a function ofradial distance from the axis in a manner such that the curvature of thecathode surface decreases with increasing radial distance from the axis.Inthis manner electrons emitted from the regions toward the periphery ofthe concave cathode are focused, in the absence of a magnetic field,toward the same point on the axis as are electrons emitted from portionsof the cathode near the axis. The increased curvature near the axiscauses these latter electrons to be emitted with a greater inwardvelocity than would be the case with a conventional spherical cathodesurface. This initial and extra radial velocity compensates for theeffect of the more peripheral electrons being bent outwardly less in thelens, due to the spherical aberration suffered by conventional electronguns.

In a preferred embodiment of an electron gun of the present invention afocusing electrode is placed sym- "metrically about the convergingelectron stream between the curved cathode surface and the anode. Thefocusing electrode is asi'ngle, unipotential electrode which is shapedto create fields in the region external to the converging beam,satisfying proper boundary conditions at the beam edge so that theelectrons in the beam behave as though the beam and its space chargecontinue to a far greater distance. The design of the shape of thefocusing electrode, as well as that of the other electrodes in the gun,is carried out by means of an electrolytic tank analog of that part ofthe gun structure external to the beam in the cathode-anode region.

'trons.

In another example, a hollow cylindrical convergent beam of chargedparticles is provided about an elongated axis by a gun having a sourceof particles in a toroid-like figure of revolution about the elongatedaxis. The cross section of the emissive surface which is rotated aroundthe axis in a curve symmetric about a line parallel to the axis. Thecurvature of this curve decreases as a function of distance from theline.

The novel features of this invention, as well as the in vention itself,both as to its organization and method of operation, will best beunderstood from the following description, taken in conjunction with theaccompanying drawings, in which like reference numerals refer to likeparts, and in which:

FIGURE 1 is a selected diagram of a traveling-wave tube of the priorart; FIG. 2 is a series of graphs of electron density taken across thescalloped electron beam of FIG. 1; each of the individual plots beingdirectly below the point in the beam of FIG. 1 where it represents thebeam cross section;

FIG. 3 is a diagram illustrating electron trajectories in the gun of thetube of FIG. 1;

FIG. 4 is a graph illustrating spherical aberration in electron guns;

FIG. 5 is a diagram illustrating the noncircular curvature of thecathode in a gun of the present invention;

FIG. 6 is a graph illustrating curvature of the cathode of FIG. 5 as afunction of radial distance oif axis;

FIG. 7 is a sectioned diagram of a traveling-wave tube utilizing theelectron gun of the present invention;

FIG. 8 is a series of graphs illustrating the electron density acrossthe stream of the tube of FIG. 7; the individual graphs again beingdirectly below the point of the beam where they represent the crosssection of the beam;

FIG. 9 is a diagram illustrating another nonspherical cathode of theinvention;

FIG. 10 is a graph illustrating curvature of the cathode of FIG. 9; and

FIG. 11 is a schematic diagram of a convergent, hollow cylindricalcharged particle gun in accordance with the present invention.

Referring more particularly to FIG. 1, an electron gun 10 is shown asutilizing a thermally emissive cathode 12 which is heated by a filamentheater 14. The heater 14 is energized by a voltage source, not shown.The cathode 12 has a circularly or spherically curved emissive surface16 which emits an initially converging stream 18 of elec- A focusingelectrode 20 aids in the initial beam formation by compensating alongthe boundaries of the beam in the cathode-anode region for space chargeeffects in the beam. An anode 22 is disposed adjacently to anddownstream from the focusing electrode 20. The anode 20 has a centralaperture 24 to permit passage of the electron stream 18 out of theelectron gun. The electrodes of the gun 10 are connected to a source ofpotential 26 to provide them with appropriate operating potentials. Theelectron stream 18 after emerging from the anode aperture 24 enters theenvironment of an axial focusing magnetic field B. In this regard it maybe noted that the anode 22 may be of a ferromagnetic material in orderto shield the cathode-anode region from this magnetic field. The axialmagnetic field is produced by a magnet 28 which may be an electromagnetor a permanent magnet.

The shading in the electron stream 18 of FIG. 1 makes apparent thescalloping of high density portions of the stream. The first graph 30 ofFIG. 2 illustrates the variation of electron density across the streamas it emerges from the anode aperture 24. It may be seen that the highdensity peaks occur near the outer periphery of the beam. It may also beseen that the electrons in these high density portions of the beam havean inward component of velocity so that slightly further down the streamwhere its cross section is represented by the graph 32 the high densityregions of the beam are closer to the axis. At the point represented bygraph 34 the high density regions are yet closer to the axis and is thepoint in the first scallop where the high density regions are closest tothe axis. Here the space charge repulsion forces associated with theelectrons begin to take over and force the high density portions of thebeam to expand radially outward, as illustrated by the graph 36. At thepoint in the stream corresponding to the graph 38 the high densityelectrons have reached their maximum excursion from the axis of thestream and again the magnetic field B begins to constrain them and forcethem toward the center. Graph 42 illustratesthe next in the scallopingamplitude of the high density portions of the beam while graph 44represents the succeeding maximum. Graph 46 illustrates the next maximumand also shows the extent to which the beam itself, as well as the highdensity regions within the beam, has spread. This of course representsdefocusing of the stream and points up the requirement that theinteraction structure, represented by the dotted lines 47, must beplaced at a wastefully great distance from the scalloped electron streamin order to prevent melting or other destruction due to electroninterception. The dotted lines 49 illustrate the lower density portionsof the stream comprising, for the most part, electrons emitted from theless peripheral portions of the cathode. It is to be noted that thisportion of the beam is also scalloped and that the relative phase of thescalloping is such that the denser portion sometimes passes outside ofthe less dense portions. This is illustrated to some extent by the graph38.

FIG. 3 illustrates a number of individual electron trajectories from thecathode of the gun of FIG. 1 to the point on the axis to which theelectron would be focused in the absence of space charge effects and aconstraining magnetic field. The individual electron trajectories arelabeled i, ii, iii, iv, v, vi, vii and viii, with i representing anelectron emitted from near the periphery of the circularly orspherically curved ernissive surface 16, While viii represents anelectron emitted near the axis. The spherical aberration in the lensbetween the cathode 12 and the anode 22 mainfests itself in causing theouter electrons to be bent less outwardly than they would be if the lenswas a pure spherical lens without aberration. This results in theelectron trajectory i intercepting the axis at a far shorter focallength than the electron trajectory viii. A nonhomocentric beam isdefined as having such properties. The fact that the electrons do nothave a common focal length precludes the selection of a magnetic fieldstrength B which will provide ideal Brillouin flow for the entire beam.It also causes the bunching of the electrons near the outer periphery ofthe beam in the region 48 near the anode aperture 24. Crossing-over ofthe electrons in this region gives rise to the sharp density peaks onthe graph 30 of FIG. 2. It may also be seen in observing the group oftrajectories in the region 43 that these trajectories have aconsiderably greater radial inward component of velocity than the innerportions of the beam and, as discussed above, this is one of the factorsin the undesirable beam scalloping illustrated in FIGS. 1 and 2.

FIG. 4 is a graph which is useful in representing the degree of excessslope in the region 48. The observed slope of electrons emitted from aprior art electron gun at the anode aperture 24 is plotted in thisgraph. If the lens was purely without aberration, the slopes of theelectrons would increase linearly with radial distance from the axis tothe diameter of the anode aperture. In FIG. 4 this condition isrepresented by the straight line 50, while the curve line 52 representsthe observed values. The region 48 of FIG. 3 is represented on FIG. 4 bythe bracket near the upper end of the curve 52. It may also be seen inthe region from .04 inch to .06 inch in the axis that there is excessslope to the trajectories, while below .03 inch the slope is somewhatless than ideal. To restate one of the objects of this invention, it isdesired to provide an electron gun where the actual plot of tra jectoryslope is a straight line as represented by the line 5% in FIG. 4.

As stated earlier, the scalloping and the uneven distribution ofelectrons across the beam is caused by the electrons emitted from theouter regions of the cathode being not bent suificiently outwardly bythe cathode-anode lens. In other words, with respect to electronsemitted near the periphery, they have too great a radially inwardvelocity. In FIG. 5 it is illustrated how this excess inward velocity ofthe outer electrons is corrected in the electron gun of the presentinvention. An electron gun 54 having a cathode 56 is illustrated. Thecurved emissive surface 58 is not spherical or circular but has acurvature which increases near the axis or center line of the gun. Thevariation of the curved surface 58 from a circular surface is shown bycomparing it with a circle shown by the dotted line 60. At the peripheryof these two surfaces 58 and 60 their curvatures are equal but near thecenter line the curvature of surface 58 is greater than that of surface60. Thus, electrons which are emitted near the periphery of the cathode56 have approximately the same radial veloc ity as those emitted from acircular cathode and thus would focus like the electronindicated by thetrajectory i of FIG. 3. However, electrons emitted near the axis have agreater inward radial velocity than those of the prior art guns and arethus caused to focus with a shorter focal length than if they wereemitted from a spherical surface and thus the convergent beam may be ahomocentric one.

FIG. 6 is a plot of the curvature of the emissive surface 58 as afunction of radial distance from the axis. On the abscissa, R representsthe outer edge of the cathode surface 58. The 'line 62 is straight andis parallel to the abscissa representing that in a circular or sphericalsurface the radius of curvature, or the curvature defined as thereciprocal of the radius of curvature, is constant across the sphericalsurface. Line 64 represents the curvature of the surface 53 and it isseen that the curvature is greater near the axis than at its periphery.It is in fact equalto the curvature of the spherical cathode at theperiphery and significantly less than at the axis. I

Referring to FIG. 7, a portion of a traveling-wave tube 66 is shownwhich utilizes the electron gun 54 of FIG. 5 The cathode 56 has a curvedemissive surface 58 which varies from the spherical, as may be seen bycomparing it to the dotted circular line 60. A focusing electrode 68 isshown in place to aid in the initial beam formation, while aferromagnetic anode 70 is illustrated with an anode aperture 72 throughwhich an electron stream 74 emerges. After the stream emerges from theaperture 72 it enters the environment of an axial focusing magneticfield B which is produced by an electromagnet 76. Be cause of the goodfocusing and collimation of the electron gun 54 the stream mayeffectively be placed much closer to the interaction structure 78 whichis represented by dotted lines.

FIG. 8 illustrates the improved electron density distributions acrossthe stream 74 by a series of individual plots or profiles 80, 82, 84,86, 88, 90, 92, 94 and 96 which may be compared to those of FIG. 2 forshowing improved collimation and focusing of the electron stream 74 ascompared to that of electron stream 18 of the prior art. In particular,it may be seen from the electron density curves of FIG. 8 that thesevere high density peaks of the prior art as illustrated in FIG. 2 donot occur and the stream is greatly improved as regards its lack ofscalloping and spreading. Obviously, with electron gun 54 higher beamcurrents and higher perveance may be achieved and utilized withoutexcessive beam spreading and scalloping so that a higher energy beam maybe projected closer to an interaction structure, such as a helix, toprovide substantially increased interaction between traveling waves, forexample, and the electron stream.

FIG. 9 illustrates a curved, noncircular cathode of the presentinvention which is alternative in its manufacture to that shown in FIG.5. In the cathode 97 of FIG. 9 the curvature of the emissive surface 98in its central portion near the axis is substantially circular orspherical as may be seen by comparing it to the circular dotted line100. Near the periphery of the curved surface, however, the curvature isdecreased and the emissive surface falls away from the circular line 100which may be taken as representing a prior art cathode.

The graph of FIG. 10 relates to the structure of FIG. 9

and illustrates the curvature of surface 98 by line 102 which may bereadily compared with the line 104 which shows the constant curvature ofthe circle 100. The curvatures are equal near the axis and diverge atradial distances toward the edge of the cathode represented by R on theabscissa. The result is substantially the same as with the nonsphericalcathode of FIG. the outer electrons are given less radially inwardvelocity to make possible better collimation of the beam.

FIG. 11 illustrates a convergent, hollow cylindrical charged particlegun of the present invention. Here, the

emitter surface 106 has the same general properties as those of thepreviously described guns, for example, that of FIG. 5. That is, theemissive surface 106, or a cross section taken through it, is a curvesymmetrically disposed about a line 108 which is substantially parallelto the center line of the gun. Here again, the curvature of the curve ofthe emissive surface 106 decreases as a function of distance from theline 108. The emissive surface 106 is a toroid-like figure of revolutionabout the center line or axis of the gun. A toroidal focusing electrode110 is shown disposed adjacent the cathode to aid in forming aconvergent annular beam of charged particles. An annular acceleratingelectrode 112 is positioned next downstream from and adjacently to thefocusing electrode.

As set forth above the scope of the invention includes devices forforming beams of charged particles other than electrons, for example,ions or protons or the like. Further, the invention also includes gunsfor producing other than beams of cylindrical form, for example, FIG. 5illustrates a gun which may provide a planar or sheet beam in which casethe (12 reference is not a single line axis but is a longitudinalreference plane lying perpendicular to the plane of the drawing.

I claim:

1. A charged particle gun for producing a well-collimated stream ofcharged particles along a linear path comprising: a charged particleemitter having a concave equipotential charged particle emissivesurface, said emissive surface being a non-spherical figure ofrevolution having an axis of revolution coincident with said path, theintersection of said surface with a plane containing said axis defininga curve extending continuously from one side of said axis to the other,the radius of curvature of said curve increasing as a function oftransverse distance from said axis to said curve, and means for focusingthe charged particles emitted from said surface into a wellcollimatedstream along said linear path.

2. In an electron gun for emitting a Well-collirnated electron streamalong a predetermined linear path, a concave equipotential electronemissive surface defining a non-spherical figure of revolution having anaxis of revolution coincident with said path, the intersection of saidsurface with a plane containing said axis defining a curve extendingcontinuously from one side of said axis to the other, and the curvatureof said curve decreasing as a function of transverse distance from saidcurve.

3. A charged particle gun for emitting a Well-collimated, hollow,cylindrical stream of charged particles along a predetermined axiscomprising: a curved equipotential charged particle emissive surface,said emissive surface being a toroid-like figure of revolution having anaxis of revolution coincident with said predetermined axis, theintersection of said surface with a plane containing said axis defininga pair of curves, each curve of said pair being symmetrically disposedabout a line parallel to said axis and extending continuously from oneside of said line to the other, the curvature of each said curvedecreasing as a function of transverse distance from its said line ofsymmetry, and means for focusing the charged particles emitted from saidsurface into a well-collimated, hollow, cylindrical stream.

References Cited in the file of this patent UNITED STATES PATENTS2,817,040 Hull Dec. 17, 1957 2,840,754 Linder June 28, 1958 2,921,223Birdsall Jan. 12, 1960

2. IN AN ELECTRON GUN FOR EMITTING A WELL-COLLIMATED ELECTRON STREAMALONG A PREDETERMINED LINEAR PATH, A CONCAVE EQUIPOTENTIAL ELECTRONEMISSIVE SURFACE DEFINING A NON-SPHERICAL FIGURE OF REVOLUTION HAVING ANAXIS OF REVOLUTION COINCIDENT WITH SAID PATH, THE INTERSECTION OF SAIDSURFACE WITH A PLANE CONTAINING SAID AXIS DEFINING A CURVE EXTENDINGCONTINUOUSLY FROM ONE SIDE OF SAID AXIS TO THE OTHER, AND THE CURVATUREOF SAID CURVE DECREASING AS A FUNCTION OF TRANSVERSE DISTANCE FROM SAIDCURVE.